How to attach parts of one model to another
In your case you can delete a lot of useless edge loops on your foot, you'll have the amount of vertices that will allow you to make the joint with your leg. As long as you choose the good edges it won't affect the shape of your foot. To do so you'll have to dissolve some edges, use the knife to create some edges between vertices, and at last merge the vertices of foot and leg. And don't forget to delete the face on the bottom of your leg.